1. Field of the Invention
The present invention relates to a honeycomb heater, which can be used as a preheater for control of automobile exhaust emissions.
2. Description of the Related Art
Conventionally, porous ceramic honeycomb structures have been employed as carriers for catalysts for removing, for example, nitrogen oxides, carbon monoxide and hydrocarbons present in the exhaust gas of internal combustion engines of automobiles. Recently, a honeycomb structure composed of a metallic material has drawn attention since the metallic honeycomb structure has high mechanical strength and heat resistance. The metallic honeycomb structure may be made by firing a green compact including metal powders.
As restriction of exhaust emission controls has been intensified, there has been a demand for development of heaters adapted to decrease pollutants during the initial running of the engine when the engine is not warmed up.
U.S. Pat. No. 5,063,029 discloses a resistance adjusting type heater including an electrically conductive honeycomb structure, at least two electrodes in electrical contact with the honeycomb structure, and means for adjusting electrical resistance, such as a slit, provided in the honeycomb structure hereby the heater can regulate its heat generation. The heater reduces pollutants during the initial running of the engine.
Sometimes a position of the heater installed in the exhaust system of an automobile may become important. When the heater is arranged below the floor of an automobile, the heater is not subject to severe conditions, and the heater does not develop any problems. However, when the heater is disposed at a position close to the engine, for example, in a manifold in the exhaust system, the heater is subject to severe conditions including high temperatures and frequent large temperature changes, and partition walls of the honeycomb structure thereof may sometimes deform or crack due to thermal stress thereby increasing electrical resistance.
Moreover, a honeycomb structure of the heater in U.S. Pat. No. 5,063,029 may have a wide range in a number of passages in one square inch from 6 to 500, and the honeycomb structure has room for improvement in improving thermal stress.